I'm no expert, but I don't see how you can do a 72 hour fast without losing muscle. I also can't imagine what the benefit might be as compared to less severe fasts.
btw I've rather easily fallen into a routine of 16 hour IMF, 5 days/week. I'm naturally a moderate, I guess, but this is working out pretty well for me. Also, on
Carbonfiberboy 's advice, I've been enjoying whey protein shakes after my long weekend rides.